Radio frequency (RF) to digital polar data converter and time-to-digital converter based time domain signal processing receiver

ABSTRACT

The present disclosure includes a time-to-digital converter (TDC) based RF-to-digital (RDC) data converter for time domain signal processing polar receivers. Polar data conversion achieves better SNR tolerance owing to its phase convergence near the origin in a polar coordinate. The proposed RDC consists of a TDC for phase detection and an analog-to-digital converter (ADC) for amplitude conversion. Unlike the conversional data converter, the proposed ADC&#39;s sampling position is guided by the detected phase result from the TDC&#39;s output. This TDC assisted data-converter architecture reduces the number of bits required for the ADC. In addition, oversampling is no longer needed. With precisely controlled tunable delay cells and gain compensator, this hybrid data convertor is capable to directly convert Quadrature Amplitude Modulation (QAM) waveforms and Amplitude Phase Shift Keying (APSK) waveforms directly from the RF signal without down-conversion. Thus, the proposed RDC architecture achieves lower power consumption and better performance comparing with conventional I/Q receivers.

CROSS-REFERENCE TO RELATED APPLICATION(S)

The present application claims priority to U.S. Provisional ApplicationNo. 62/584,324, titled TDC BASED HYBRID POLAR DATA CONVERTER AND TIMEDOMAIN SIGNAL PROCESSING FOR RECEIVERS, filed Nov. 10, 2017, thecontents of which are incorporated herein by reference in its entirety.

FIELD

The present invention relates to wireless radio receiver architecture.Particularly, the invention relates to the time to digital converter(TDC) based polar receiver.

BACKGROUND

There is an increasing interest to push mixed-signal/digital operationsinto RF front-ends. “Digital-RF” has stimulated much attention inresearch and commercial applications. Digitally assisted RF blocks canretain the full flexibility of various digital techniques such asdigital waveform synthesis, digital modulation, digital signalprocessing, digital pre-distortion, digital calibration, digitalself-healing and digital signal amplification. In order to handleversatile wideband complex modulations with high efficiency, softwaredefined radios (SDR) need to be reconfigurable as far as itsarchitecture concerned. Among various transmitter architectures, polartransmitter suffers from bandwidth expansion problems when convertingdata from I/Q to polar, while a digital I/Q transmitter has the largestpower combining loss of 3 dB. As a compromise, a transmitter withmulti-phase carriers can achieve small bandwidth expansion and low powercombining loss, if multi-phase carriers can be generated with low power.

For wireless applications, “Digital-RF” can potentially obtain highprecision, high dynamic range, low noise, and high power efficiency thantheir analog counterparts. Moreover, it retains the full flexibility ofvarious digital techniques, e.g., direct digital modulation, digitalsignal processing, digital pre-distortion, digital waveform generation,digital calibration, digital assisted self-healing, digital poweramplification and most importantly suitability for digital synthesisdesign flow, all of which will benefit future wireless transceiverdesigns. Semiconductor manufacturing has advanced tremendously in recentyears such that it is possible to apply many traditionallydigital-signal processing techniques to analog circuits. Analog circuitdesign flow can be greatly improved, if the following aspects can berevolutionized: transceiver architecture, digital assisted analogcircuit and analog circuit synthesis.

SUMMARY

The system of the present application features a TDC based hybrid polardata converter 40 (FIG. 4) and time domain signal processing procedurecorresponding to this data convertor (FIG. 6, 7) for wirelesscommunication receiver converting a received baseband signal to adigitized data, and then further demodulating. The received analogbaseband signal is split into two branches and is fed to both the TDCand ADC. The TDC path measures the signal's phase information, while theADC is controlled by the TDC's output and captures the signal'samplitude. With the sampling position control mechanism, the ADC'snumber of bits can be greatly reduced by comparing it with aconventional I/Q topology, and both the ADC and TDC can sample thesignal at the baseband signal's frequency without oversample. Multipleprecisely controlled tunable delay cells and gain compensation blocksare inserted in the architecture to form a time domain signal processingand demodulation process. A high data throughput 1024-APSK modulationsignal is enabled with the proposed receiver.

The system of the present application includes a time processing basedtransceiver in the backs and architecture that doesn't follow thetraditional amplitude dependent waveform transmission and receiving.Instead, the frequency and phase information are extracted such that therequirement for amplitude detection is much relaxed, resulting ingreatly reduced number of bits (NoBs) and power required foranalog-to-digital converter (ADCs). With advanced technology, frequencyand phase information can be accurately detected using time-to-digitalconverters (TDCs) that require much lower power. In addition, the systemincludes a multi-phase-carrier digital transceiver architecture with abroadband digital power amplifier (DPA) and all digital phase-lockedloop (ADPLL) system, which can be adaptively reconfigured as an I/Q, amulti-phase carrier, or a polar transceiver for digital communications.The integrated ADPLL system takes the baseband information andsynthesizes the desired baseband control signals independently for eachDPA path, eliminating additional power hungry digital-to-analogconverters (DACs).

Cartesian I/Q transceivers are widely used in nowadays wirelesscommunication systems. Current wireless communication standards, such asGSM, WLAN and LTE, all require a large peak to average ratio, whichleads to a large burden to the circuits on the I/Q signal path. Topreserve the signal's peak to average ratio, those circuits need to havea high linearity. Namely, a large power consumption is required for I/Qtransceivers. FIG. 1 gives a normal I/Q wireless transceiverarchitecture. Because the I/Q signals have varying envelope, all thebuilding blocks in the signal path require a high linearity, which leadsto a high power consumption level.

The high peak to average ratio in an I/Q architecture is due to thechange of the signal amplitude. One solution to relieve this issue is toconvert the I/Q signal into polar signal. In the polar architecture, thephase signal path has constant amplitude. Thus, there is no linearityrequirement. The amplitude information can be added to the signal at thePA stage, the last stage of the transmitter, by using a DPA structure. Awell-developed polar direct modulation transmitter architecture ispresented in FIG. 2.

Polar transmitters have been in use and studied for decades. However,polar receivers have barely been studied during this time frame. One ofthe reasons for this lack of study is that the received signal becomesvery tiny when it goes through a long distance transmission to thereceiver. Although the linearity requirement is not as high as thetransmitter part, the main burden of a receiver has been shifted to theADC part and baseband signal processing part. For a conventionalwireless local area network (WLAN) receiver, the system calls for an ADCfor each I and Q signal path. These ADCs require at least 10 number ofbits and running at a minimum oversample ratio of 4, which is around 100MS/s ADC sample rate. The ADC specifications mentioned above arefeasible, but with large power consumption and cost.

One reason to pursue a polar receiver is that comparing with aconventional Cartesian I/Q receiver, a polar receiver requires less dataconverter bits when dealing with the same signal-to-noise ratio (SNR)level signal. FIG. 3 explains this advantage in an intuitive way. Thefirst plot 10 illustrates a Cartesian I/Q conversion space 10. Thedashed lines 12 indicate data converters' quantization level. In thisexample, the signals in both the I and Q axes are quantized into twelvelevels. In the polar case as shown in second plot 20, the space has beenquantized with sixteen phases and six amplitude levels by the dashedlines 22. Note that no matter dealing with large or small input signal,the I/Q converter's quantization resolution (small box) in plot 10remains the same, while the polar converter plot 20 has largequantization resolution when the input signal is large and has finerquantization resolution when the signal has a small amplitude. In otherwords, the polar data converter's quantization steps are automaticallyadjusted based on input signal's amplitude, leading to improved SNR withthe same number of bits for data converters. Because all the phases willfinally converge to the origin, the quantization resolution will beinfinite 1× magnified when the signal approaches zero. The last figureplot 30 in FIG. 3 shows a simulation comparison between I/Q and polararchitecture when dealing with 16QAM and 64QAM signals. We can see thatthe polar architecture achieves much better results. In our proposedarchitecture for 16-QAM, we need only a two-bit ADC, which has a largerquantization step and noise margin, leading to improved noise tolerance,namely, less SNR requirement. This simulation shows the improveddetection in noisy channel conditions. Since the polar receiver requiresless number of bits for data conversion, it is a possible increase theintermediate frequency (IF) frequency. With a higher IF frequency, thedesign complexity of a receiver RF frontend can be reduced when dealingwith image rejection and other issues.

BRIEF DESCRIPTION OF THE DRAWING(S)

The above and further advantages of this invention can be betterunderstood by referring to the following description in conjunction withthe accompanying drawings, in which:

FIG. 1 shows a prior art embodiment of a traditional I/Q wirelesstransceiver architecture;

FIG. 2 shows a prior art embodiment of an existing polar directmodulation TX architecture;

FIG. 3 shows a comparison between a Cartesian I/Q converter and polarconverter;

FIG. 4 shows a block diagram of the TDC based hybrid polar dataconverter for use in a receiver;

FIG. 5A shows the polar RX operational principle for conversion of a16-QAM baseband signal which may be used in accordance with the systemof the present application, FIG. 5B gives the constellation of the16-QAM modulation as well as its amplitude and phase quanta;

FIG. 6 shows a polar RX operational principle for conversion of a 64-QAMband limited modulated RF signal which may be used in accordance withthe system of the present application;

FIG. 7 shows a complete direct-RF polar transceiver system which may beused in accordance with the system of the present application;

FIG. 8 shows a comparison between QAM and APSK modulated signals andtheir number of bits requirement;

FIG. 9 shows 64-QAM and 64-APSK (2-bit amplitude and 4-bit phase)modulation constellations;

FIG. 10 shows a rectangular 64-QAM and a polar 64-APSK constellationswith added phase noise;

FIG. 11 shows a rectangular 64-QAM and a polar 64-APSK constellationswith nonlinear distortion;

FIG. 12 shows a comparison of the BER vs. SNR performance of therectangular 64-QAM signal and the proposed polar 64-APSK signal withadded phase noise;

FIG. 13 shows a comparison of the BER vs. SNR performance of therectangular 64-QAM signal and the proposed 64-APSK signal withnonlinearity distortion and a comparison of measured BER versus phasenoise and SNR for 64-QAM, 1024-QAM, 64-APSK, and 1024-APSK modulations.

DETAILED DESCRIPTION OF THE DRAWING(S)

In the present description, certain terms have been used for brevity,clearness and understanding. No unnecessary limitations are to beapplied therefrom beyond the requirement of the prior art because suchterms are used for descriptive purposes only and are intended to bebroadly construed. The different systems and methods described hereinmay be used alone or in combination with other systems and methods.Various equivalents, alternatives and modifications are possible withinthe scope of the appended claims. Each limitation in the appended claimsis intended to invoke interpretation under 35 U.S.C. § 112, sixthparagraph, only if the terms “means for” or “step for” are explicitlyrecited in the respective limitation.

The present application includes a novel wireless receiver architecture,which uses both TDCs and ADCs to form a polar data conversion topology.A block diagram of an exemplary architecture of a TDC based hybrid polardata converter 40 is shown in FIG. 4. The antenna 42 captured RF signal44 is directly down converted to baseband frequency or an IF. Direct-RFsampling is possible if the carrier frequency is within the operationalrange of the circuits. Instead of splitting into I/Q, the RF signal isfed into TDC path 46 and ADC path 48 directly. In the TDC path 46, thesignal will go through a hysteresis buffer 50, which preserves signal'sphase information and removes its amplitude. By carefully adjusting thethreshold voltages V_(TH) and V_(TL) of the buffer, the hysteresiseffect will reduce the noise in phase domain. The phase information ofthe received modulated signal is resolved with a reconfigurable m-bitTDC 52 by detecting the time difference between the zero-crossing ofreference clock and the zero-crossing of the modulated signal. Thereference clock of the receiver is synchronized to transmitter based ona feedback loop 54 provided by digital baseband 56. The feedback loop 54selects the closest phase generated by a multiphase baseband clockgenerator 58 and further fine tunes the selected phase by adjusting thedelay of a digital to time converter (DTC) 60. The use of multi-phaseclock will relax the tuning range of delay cell and reduce powerconsumption. The amplitude of the received modulated signal is capturedby a reconfigurable n-bit ADC 62 with the aid of TDC captured phaseinformation. The ADC sampling clock 64 is modulated by TDC measuredphase information 66 through another DTC 67 and a multiplexer formed ADCsampling position adjustment module 68. After the conversion, theoutputs of ADC and TDC represent the amplitude 65 and phase 69 of thebaseband signal. Thus, the proposed architecture can also replacedigital domain CORDIC processing modules in the digital baseband.

FIGS. 5 and 6 illustrate the phase and amplitude principles 70 of thedescribed TDC based polar receiver. FIGS. 5A and 5B illustrate anexample of a 16-QAM unfiltered baseband signal, where there are twelvedifferent phase values 72 and only three different amplitude levels 74on the constellation plot. FIG. 5A shows the baseband signal 78 in timedomain. In a traditional I/Q receiver, the baseband signal 78 would besplit into I and Q path and digitized by two ADCs. However, the samplerate of the ADCs needs to be much higher than the baseband signal 78frequency, in order to gather enough data for the digital basebandprocessor to recover the phase and amplitude information of the signal.

By examining the constellation of the 16-QAM signal 78 in FIG. 5B, wefind out that instead of using two 10 bits ADCs, a 4-bit TDC may be usedto obtain the phase information and a 2-bit ADC to identify theamplitude. The data obtained by TDC and ADC is sufficient to demodulateand restore the constellation. Furthermore, the TDC and ADC are able torun at baseband signal frequency, which greatly reduces the dataconverter's design complexity.

The baseband waveform 78 of a 16-QAM signal shown in FIG. 5A alsodemonstrates the proposed polar receiver's working principle. Taking‘Symbol N’ as an example, initially, the baseband phase locked loop(PLL) frequency needs to be locked to the baseband signal's frequencyand aligned the phase to symbol start point. The TDC begins at the starttime of a baseband symbol and stops at baseband signal's risingzero-crossing point. The TDC's output represents the phase informationof the signal. Then the amplitude needs to be determined. The amplitudeof a sinusoid waveform is its maximum amplitude. Taking a closer look ofFIG. 5A, the maximum (Amp. 1) occurs at the position of risingzero-crossing point with a λ/4-time delay, as indicated with a solidhorizontal line. Thus, if we let the ADC sample the value at that point,we can obtain the amplitude of the signal. With the gathered phase andamplitude data, the baseband signal can be recovered and theconstellation restored. Still referring to FIG. 5A, the ‘Symbol N+2’ and“Symbol N+5” are two additional examples to illustrate the phase andamplitude conversions.

Different from conventional receivers, these two data converters aresampling the signal at baseband signal frequency without oversampling,which is around 10 to 20 MHz for 802.11-a/b/g/n wireless protocols. In atraditional polar transmitter, there is significant bandwidth expansionthrough I/Q to polar conversion. Conventional ADCs blindly convert thereceived waveform to digital data, thus needing oversampling. Thepresent time-processing based receiver samples the signal's phase andamplitude information once every symbol period for demodulation. Thus,there is no need to do oversampling.

Normally, multiple filters are applied to the upconverted signallimiting its bandwidth to fit transmitter's requirements. FIG. 6presents a comparison between a baseband signal before and after abaseband low pass filter “Filtered Signal” vs. “Unfiltered Signal”. In aQAM baseband signal, there will be a sudden phase and amplitude changebetween each symbol 82. This abrupt phase and amplitude jump willintroduce high frequency components. The high frequency component needsto be filtered in order to pass a transmission mask defined in wirelesscommunication protocol. With the absence of the high frequency componentthe signal will be distorted and lead to a higher (Error vectormagnitude) EVM level. However, also from FIG. 6, it is shown that thesignal in the edges of a symbol period 84 is less distorted than thecenter of a symbol. Thus, it is more reliable to sample both phase andamplitude information around the edge of a symbol where is lesssusceptible to the filter.

There is another issue during the ADC sampling position adjustment.Referencing back to FIG. 5A and ‘Symbol N+2’. In this symbol period therising zero-crossing point has already exceeded more than ¾ cycle of thesymbol. If we further delay the ADC's sampling position by λ/4, then theADC will sample a value belonging to the next symbol or outside of thesuitable sampling window in the situation presented in FIG. 6. Toprevent this kind of situation, the time difference between system clockand falling zero-crossing point is measured by another TDC and used asthe phase information. Then the ADC's sampling position is set λ/4 aheadof the falling zero-crossing point.

Together with existing polar transmitters, a completed wirelessdirect-RF polar transceiver system 90 is formed as shown in FIG. 7 thatcan deal with most of existing digital amplitude and phase modulations.The left part of the FIG. 7 presents an abstracted diagram of a welladopted direct-RF polar transmitter 92 architecture, while the rightpart gives the abstraction of the present polar receiver 94architecture. This intelligent control of sample point allows us tosimplify the architecture of data converters.

The next generation wireless standards call for highly complexedmodulations in order to achieve high data throughput. Complexmodulations such as 256-QAM and 1024-QAM put stringent requirements onthe phase noise of the PLL, the linearity of the PA and the sample rateas well as the dynamic range of the ADC. Even with the best effort, thestate-of-the-art PLLs and PAs can barely support the high-densitymodulations such as 1024-QAM, leaving no margin for tolerance of othersystem impairments such as IQ and gain mismatches encountered inconventional Cartesian I/Q transceivers. However, these requirements aremuch relaxed with the constellations arranged in polar coordinates. Apolar based 64 amplitude and phase shift keying (64-APSK) modulation ispresented in FIG. 8 and compared with commonly used I/Q Cartesian based64-QAM modulation. The error span due to phase noise and nonlineardistortion is proportional to the distance from the origin to theconstellation point. For Cartesian based QAM modulation, the outerconstellation points are much more sensitive to phase variations andnonlinear distortions. On the other hand, APSK can tolerate the sameamount of phase noise regardless of its amplitude and thus has betterefficiency and impairment tolerance for implementation in a polarsystem. In addition, the APSK modulations requires less number of bit inboth amplitude and phase information in the presented polar transceiversystem compared to traditional QAM modulation in the polar system asshown in FIG. 8.

Simulations and Measurements were taken to compare the performancesbetween Cartesian QAM modulations and APSK modulations using theproposed direct RF-sampling polar direct-RF receiver in presence ofcommonly seen impairments such as phase noise and nonlinear distortion.FIG. 9 and FIG. 10 show the simulated constellations of 64-QAM and64-APSK modulated signals with the present of phase noise and nonlineardistortion, respectively. It can be clearly seen that less constellationpoint overlap can be found in 64-APSK modulation compared to 64-QAMmodulation. FIG. 11 and FIG. 12 present the theoretical and simulatedbit error rate (BER) results versus phase noise and nonlinear distortioncharacterized as SNR, respectively. APSK modulation achieves better SNRtolerance in both scenarios.

Measurement results are presented in FIG. 13. Transmitted modulatedsignals from a signal generator source are presented together with themeasured reconstructed digital data points of the direct-RF polarreceiver output in FIG. 13. BER testing results were measured to comparedifferent complex modulations. As expected, the APSK outperforms theCartesian QAM modulation when phase noise and nonlinear distortion inform of SNR are presence. To achieve the same BER, 1024-APSK relaxes thephase noise by 6 dB and the SNR by 8 dB, respectively, comparing to itsCartesian 1024-QAM counterpart.

The embodiments of the invention described above are intended to beexemplary only. The scope of the invention is therefore intended to belimited by the scope of the appended claims.

In the foregoing description, certain terms have been used for brevity,clearness, and understanding. No unnecessary limitations are to beinferred therefrom beyond the requirement of the prior art because suchterms are used for descriptive purposes and are intended to be broadlyconstrued. The different configurations, systems, and method stepsdescribed herein may be used alone or in combination with otherconfigurations, systems and method steps. It is to be expected thatvarious equivalents, alternatives and modifications are possible withinthe scope of the appended claims.

What is claimed is:
 1. A time-to-digital converter-based hybrid polardata converter (converter) for a polar receiver, comprising: atime-to-digital converter (TDC) with a reconfigurable temporalresolution and a variable number of bits input, wherein the TDC detectsphase information of a received signal, wherein the received signal is amodulated signal on an RF signal; a low power analog-to-digitalconverter (ADC), wherein the ADC detects an amplitude information of thereceived signal, wherein an ADC sampling position for detecting theamplitude information is determined by the received phase information; afirst tunable temporal delay cell configured in an ADC path and a pluralpath of fixed temporal delay cells located in the ADC path, wherein thefirst tunable temporal delay cell is controlled by the TDC's output toprecisely set the ADC sample position at a maximum of a symbol period;and a digital domain one cycle delay coupled to the output of the TDC toalign the TDC's output with the ADC's output, such that the outputsinclude digitalized phase and amplitude data.
 2. The converter of claim1 further comprising a hysteresis buffer copied to an input of the TDC,wherein the hysteresis buffer eliminates signal amplitude information.3. The converter of claim 1 further comprising a baseband clocksynthesis module with a multi-phase output, wherein the module iscontrolled by a feedback signal, the feedback signal received from adigital baseband and the feedback signal is used to synchronize a localclock phase with the received signal.
 4. The converter of claim 3further comprising a multi-phase selection multiplexer coupled to anoutput of the baseband clock synthesis module, wherein the multiplexeris controlled by the digital baseband and selects a closest phasegenerated by the synthesis module for an approximate local clocksynchronization.
 5. The converter of claim 4 further comprising a secondtunable temporal delay cell coupled to an output of the multi-phaseselection multiplexer, wherein the second tunable temporal delay cell iscontrolled by the digital baseband and further controls the local clockphase for fine local clock synchronization.
 6. The converter of claim 1,where for a 16-QAM and a 64-APSK modulation signals, a minimum requirednumber of bits is four (4) for the TDC and two (2) for the ADC.
 7. Atime-to-digital converter-based hybrid polar data converter (converter)for a polar receiver, comprising: a time-to-digital converter (TDC) witha reconfigurable temporal resolution and a variable number of bitsinput, wherein the TDC detects a phase information of a received signal,wherein the received signal is an analog signal on an RF signal; a lowpower analog-to-digital converter (ADC), wherein the ADC detects anamplitude information of the received signal, wherein an ADC samplingposition for detecting the amplitude information is determined by thereceived phase information; a hysteresis buffer copied to an input ofthe TDC, wherein the hysteresis buffer eliminates signal amplitudeinformation; a baseband clock synthesis module with a multi-phaseoutput, wherein the module is controlled by a feedback signal, thefeedback signal received from a digital baseband and the feedback signalis used to synchronize a local clock phase with the received signal; amulti-phase selection multiplexer coupled to an output of the basebandclock synthesis module, wherein the multiplexer is controlled by thedigital baseband and selects a closest phase generated by the synthesismodule for an approximate local clock synchronization; a first tunabletemporal delay cell configured in an ADC path and a plural path of fixedtemporal delay cells located in the ADC path, wherein the first tunabletemporal delay cell is controlled by the TDC's output to precisely setthe ADC sample position at a maximum of a symbol period; a secondtunable temporal delay cell coupled to an output of the multi-phaseselection multiplexer, wherein the second tunable temporal delay cell iscontrolled by the digital baseband and further controls the local clockphase for fine local clock synchronization, and a digital domain onecycle delay coupled to the output of the TDC to align the TDC's outputwith the ADC's output, such that the outputs include digitalized phaseand amplitude data.